fix: inspector stale data warning, remove dead supported_modes code

- inspector.html: show orange '⚠ Stale: HH:MM' with tooltip when link_stats data is >15 min old (previously just showed the time with no visual warning)
- style.css: add .g-stale-warn helper class (orange, bold) for the stale indicator
- diagnose.py: remove supported_modes accumulation from parse_ethtool() — field was collected but never consumed by analyze() or displayed anywhere

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/inspector.html

@@ -428,7 +428,14 @@ function renderInspector(data) {
 
   const updEl = document.getElementById('inspector-updated');
   if (updEl && data.updated) {
-    updEl.textContent = 'Updated: ' + new Date(data.updated + (data.updated.includes('Z') ? '' : 'Z')).toLocaleTimeString();
+    const updMs = new Date(data.updated + (data.updated.includes('Z') ? '' : 'Z'));
+    const ageMin = (Date.now() - updMs) / 60000;
+    const timeStr = updMs.toLocaleTimeString();
+    if (ageMin > 15) {
+      updEl.innerHTML = `<span class="g-stale-warn" title="Data is ${Math.floor(ageMin)} minutes old — monitor may be down">⚠ Stale: ${timeStr}</span>`;
+    } else {
+      updEl.textContent = 'Updated: ' + timeStr;
+    }
   }
 
   if (!Object.keys(switches).length) {